Transmitters in many radio-based telecommunications and broadcast systems are designed to transmit wide bandwidth modulated signals with the great power efficiency. Generally, components and signal tracks of the final parts of the radio signal chain contributes significantly to the DC (Direct Current) power consumption.
A typical final amplification stage of the transmitter is made in multilayer microstrip/stripline-type printed circuit board (PCB) technology and contains an output matching network, a directional coupler and an isolator/circulator. The directional coupler picks up a small part of the transmitted signal back to a linearization/predistortion block. Microstrip transmission lines, surface mounted directional coupler and the isolator contribute to a loss of the transmitted signal and lower overall efficiency of the transmitter. All features added after the final transmitter power amplifiers are especially costly, since any insertion loss at this point has great effect on the efficiency of the transmitter. This typically implies that the final amplification stages, and especially the power amplifier, compensate by delivering a higher output power than ideally would be needed. These losses reduce the performance of the system and can increase size due to space demanding physical cooling solutions.
Efficiency enhancement techniques often involve a multi order configuration of power amplifiers (MOPA) as presented in WO 01/95481. Here, a multistage Doherty amplifier is provided. The presented system uses separate drive amplifiers for individual power amplifiers and/or groups of power amplifiers. This makes it possible to make improve efficiency and linearity by optimizing the drive for the different power amplifiers and using less drive power. Prior art solutions such as this improve upon DC/DC power consumed in the power amplifier, but may need large areas of combining networks at the output which also contributes to the loss after the power amplifier, and consequently decreases overall efficiency.
A suspended line, made of a thick metal plate and supported with quartz slabs, has been applied in high performance HEMT (High Electron Mobility Transistor) amplifier described in: R. Peter, M. V. Schneider, Y. S. Wu, High-Performance HEMT Amplifiers with a Simple Low-Loss Matching Network, IEEE Transactions on Microwave Theory and Techniques, vol. 39, No. 9, pp. 1673-1675. However, such a solution can be used only as a separate module because it is hard to integrate it with the standard PCB technology.
U.S. Pat. No. 5,896,065 presents a radially combined RF (Radio Frequency) amplifier including an input divider for dividing the input power signal into “N” input signals, and a single matching circuit configuration for matching an combining the amplified “N” input signals into one combined output power signal. The system utilizes a stripline configuration of the matching circuitry and suspends this circuitry above one surface of an aluminum chill plate. The input stage of the amplifier is disposed on the opposite surface of the chill plate with the transistors connecting the input stage with the output network across the peripheral edge of the chill plate. However, the combiner circuit is provided in a fragile structure and mechanical and electrical control of the suspended circuitry is limited.